A near-field communication (NFC) or radio frequency identification (RFID) system is a short range communication with a typical communication distance of up to 4 cm. Different from those far-field communication systems using electromagnetic transmission, NFC uses magnetic field coupling for data exchange.
Under a wide magnetic field dynamic range from 0.15 A/m up to 12 A/m, a NFC device front-end is under voltage range from less than 1V to a few tens of volts without RF limiting. For reliability and normal operation, a RF limiter is required to limit the RF input voltage of the NFC device to below 3.6V within the thick gate device's reliable operation range. At the same time, amplitude shift keying (ASK) modulation defined in NFC standards for both transmitter and receiver requires the RF limiter to be linear such that the ASK modulated signals at the NFC front-end will not be distorted.
In general, the RF limiter is required to have fast response, i.e. very short time constant, to protect the NFC device, at the same time it is required to have a slow response, i.e. a very long time constant, for not distorting the ASK modulated signal at the NFC device front-end.
A typical conventional analog limiter adopts a large on-chip capacitor for slow response to reduce distortion to ASK modulation. However, this solution may cause reliability risks due to slow response, e.g. the NFC device may be damaged due to slow response and may extend the settlement time when the field strength changes fast. Further, the conventional analog limiters can only work at active/power-up mode, cannot provide protection during power-off mode.
It is therefore desirable to provide a RF limiter which can both maintain integrity of the ASK modulated signals and protect the NFC device from being damaged due to slow response.